1. Field
The present invention relates to a lighting device including a light source and a light-guiding plate, and a liquid crystal display device using the lighting device.
2. Description of the Related Art
Liquid crystal display devices have been widely used for electronic devices such as mobile phones and personal digital assistants (PDA) because of their thinness, light weight, and small power consumption. A lighting device called a backlight is usually disposed in the liquid crystal display devices used for such electronic devices.
FIG. 1 is a schematic diagram showing an example of a conventional liquid crystal display device. As shown in FIG. 1, the liquid crystal display device includes a liquid crystal panel 10, and a backlight 20 disposed on the back surface side of the liquid crystal panel 10.
The liquid crystal panel 10 is formed of two transparent substrates 11a and 11b, as well as a liquid crystal 12 that is sealed between the transparent substrates 11a and 11b. Polarizing plates (not shown) are disposed on both sides of the liquid crystal panel 10 in a thickness direction thereof.
The backlight 20 includes light emitting diodes (LED) 21 serving as light sources, a light-guiding plate 22, a reflective sheet 23, and a prism sheet 24. The LEDs 21 are disposed at one edge surface side of the light-guiding plate 22. In the case of a 2-inch liquid crystal panel, three or four LEDs 21 are used in general.
The light-guiding plate 22 is made of a transparent resin, and is formed to have a wedge-shaped cross section as shown in FIG. 1. The reflective sheet 23 is disposed on a back surface side of the light-guiding plate 22, and the prism sheet 24 is disposed on a front surface side (liquid crystal panel 10 side).
In the liquid crystal display device configured in such a manner, light emitted from the LEDs 21 enters the light-guiding plate 22, is reflected by the reflective sheet 23, and is emitted toward the liquid crystal panel 10.
A pixel electrode is formed for each pixel in one of the two transparent substrates 11a and 11b constituting the liquid crystal panel 10, and a common electrode is formed in the other substrate to face these pixel electrodes. An amount of light transmitted through the pixels can be controlled by voltage applied between the respective pixel electrodes and the common electrode. Thus, the controlling of a light transmission amount for each pixel allows a desired image to be displayed.
In the liquid crystal display device, light emitted from the backlight 20 preferably causes an entire surface of the liquid crystal panel 10 to be illuminated uniformly. Accordingly, the fine concave and convex portions are formed on the front surface side or the back surface side of the light-guiding plate 22 to disperse the light more uniformly, or the prism sheet 24 is placed as a light distribution control plate between the light-guiding plate 22 and the liquid crystal panel 10 as shown in FIG. 1.
However, if only the LEDs 21 are disposed near the edge surface of the light-guiding plate 22 as shown in FIG. 1, uneven brightness occurs on the inner side of the light-guiding plate to cause a problem of reduction in quality of an image displayed in the liquid crystal display device. FIG. 2 is a plan view when the backlight 20 is viewed from the liquid crystal panel 10 side. As shown in FIG. 2, a plurality of LEDs 21 are generally used for the liquid crystal display device. However, if only the LEDs 21 are disposed near the edge surface of the light-guiding plate 22, light does not reach a region between adjacent LEDs 21, resulting in dark portions (portions indicated by A in FIG. 2) and portions of high luminance (portions indicated by B in FIG. 2) near the front of the LEDs 21.
Various techniques have conventionally been developed to solve the aforementioned problems. For example, a Patent Document 1 (Japanese Laid-open Patent Application Publication No. 2004-163886) discloses a lighting device in which concave lenses are disposed between a light-guiding plate and each of light sources. In this lighting device, the light emitted from the light sources is refracted by the concave lenses. Thus, occurrence of uneven brightness near the edge surface on the inner side of the light-guiding plate can be avoided. Moreover, in a Patent Document 2 (Japanese Laid-open Patent Application Publication No. 2002-357823), as shown in FIG. 3A, a light-guiding plate 26 having a semicircular notch formed in a portion corresponding to an LED 21 is described. In this light-guiding plate 26, light emitted from the LEDs 21 is refracted by the notch. Thus, the light reaches a region between adjacent LEDs 21 to prevent uneven brightness near the edge surface of the light-guiding plate.
Further, in a Patent Document 3 (Japanese Laid-open Patent Application Publication No. 2003-331628), as shown in FIG. 3B, formation of many prisms (triangular concave and convex portions) 27a in an entire edge surface of an LED side of a light-guiding plate 27 is described. In this light-guiding plate 27, light emitted from LEDs 21 is refracted by the prism 27a. Thus, the light reaches a region between adjacent LEDs 21 to suppress uneven brightness near the edge surface of the light-guiding plate.
Furthermore, as shown in FIG. 3C, there is a light-guiding plate 28 having fine concave and convex portions formed in an edge surface of its LED side. Such fine concave and convex portions are formed by blast processing with a mold block. In the light-guiding plate 28, light emitted from LEDs 21 is dispersed by the fine concave and convex portions when the light enters the light-guiding plate, and reaches a region between adjacent LEDs 21 to prevent uneven brightness near the edge surface of the light-guiding plate.
FIG. 4 is a schematic diagram showing a method of manufacturing the light-guiding plate 28 shown in FIG. 3C. As shown in FIG. 4, concave and convex portions are formed on a surface by injecting sand (abrasive grains) through a nozzle 42 to a mold block 41. At this time, concave and convex patterns can be changed by adjusting a material, a particle size, an injecting speed, an injecting amount, an injecting angle or the like of the sand. Next, the light-guiding plate 28 is molded by use of the mold block 41. Subsequently, an LED, a reflective sheet, a prism sheet and the like are mounted to the light-guiding plate 28 to constitute a backlight, and an optical characteristic (uniformity) is evaluated with the LED turned on. Then, if a desired optical characteristic is not obtained, blast processing is performed again by changing conditions.
Note that, in a Patent Document 4 (Japanese Laid-open Patent Application Publication No. 2000-356757), a projector in which two diffractive optical elements including a liquid crystal and a polymer are disposed between a light source and a lens to obtain a light beam having a uniform polarization direction is described.
However, in the lighting device using the light-guiding plate 26 shown in FIG. 3A, the LED 21 and the semicircular notch must be fairly accurately aligned with each other. The device lacks versatility because the number and positions of LEDs 21 are determined by the notch of the light-guiding plate 26. Thus, it is not easy to deal with changes in panel size.
In the same manner as the lighting device shown in FIG. 3A, the lighting device described in the Patent Document 1 has a problem that the light sources and the concave lenses must be fairly accurately aligned with each other. Since the concave lens is disposed between the light source and the light-guiding plate, there arises another problem that a large space needs to be secured between the light-guiding plate and each of the light sources whereby leakage light not entering the light-guiding plate increases, leading to a decrease in light usage efficiency.
The lighting device using the light-guiding plate 27 shown in FIG. 3B also requires the LED 21 and the prism 27a to be fairly accurately aligned with each other. A certain distance is necessary between the LED 21 and the light-guiding plate 27 to effectively use the prism 27a. Thus, leakage light not entering the light-guiding plate 27 increases to reduce light usage efficiency.
In the light-guiding plate 28 shown in FIG. 3C, fairly accurate alignment is not necessary because of the formation of the fine concave and convex portions in the entire edge surface on the LED side. However, the concave and convex portions exhibiting desired characteristics must be formed by repeating the blast processing for the mold block, the molding and the optical evaluation. Thus, there is a drawback that the mold manufacturing takes time. In the case of a mobile phone, many molds are necessary because there are many manufacturing steps. However, reproducibility of concave and convex portions formed by the blast processing is low, causing a problem of nonuniform mold qualities, which in turn causes an increase in manufacturing cost.
Further, conventional lighting devices have been designed to uniformly illuminate an entire surface of a liquid crystal panel, as described above. However, in recent years, lighting devices used for mobile phones and PDAs in particular are desired to be capable of approximately uniformly illuminating an entire surface of a liquid crystal panel, as well as to be easily designable in accordance with the desired brightness distribution, such as high brightness in the center portion or high brightness in the peripheral portions.